1. Field of the Invention
The present invention relates, in general, to a slurry for chemical mechanical polishing (hereinafter, referred to as ‘CMP’) and, more particularly, to a polishing slurry, a method of producing the same, and a method of polishing a substrate. The polishing slurry has high selectivity in terms of a polishing speed of an oxide layer to that of a nitride layer used in CMP of an STI (shallow trench isolation) process which is essential to produce ultra highly integrated semiconductors having a design rule of 256 mega D-RAM or more.
2. Description of the Related Art
CMP is a semiconductor processing technology in which a mechanical process using polishing particles between a pressed wafer and a polishing pad and chemical etching using a slurry are simultaneously conducted, and has been an essential process of global planarization technology in the production of submicron-scaled semiconductor chips since IBM Co., Ltd. in the USA developed it at the end of the 1980's.
A description will be given of a CMP process and a slurry needed for this process, with reference to FIGS. 1a to 1c. The CMP process, in which an uneven portion of a surface of a wafer is smoothed in a semiconductor process, is a process where, after the surface of the wafer is chemically transformed by an acidic or basic solution of the slurry to instantaneously form a layer weakly bonded to the surface, the layer thus formed is mechanically removed using particles in the slurry. In other words, the wafer is pressed while the slurry is provided to the surface of the wafer, thereby mechanically polishing the surface of the wafer using the particles in the slurry.
To conduct the CMP process, a head 3, on which a wafer 1 is to be mounted, a pad 4 rotating in the same direction as the head, and a slurry 2 provided between them, containing nano-sized polishing particles, are prepared. The wafer 1 is mounted on a wafer chuck 7 of the head 3 by surface tension or vacuum pressure. In the CMP process, the wafer 1 is polished by the pad 4 and the slurry 2. A polishing table 5, to which the pad 4 is attached, merely rotates, but the head 3 simultaneously rotates and reciprocates while deviating from the center of rotation of the polishing table 5. At the same time, the wafer 1 is pressed toward the polishing table 5 with a predetermined pressure. The surface of the wafer 1 comes into contact with the pad 4 due to the weight of the head and applied pressure, and the slurry flows into fine gaps in the interface, that is to say, the pores 8 of the pad. Mechanical polishing is achieved by polishing particles of the slurry and surface protrusions 9 of the pad 4, and chemical polishing is achieved by chemical components of the slurry. Furthermore, upper sides of projections of the wafer 1, in which devices are formed, first come into contact with the polishing particles or the surface protrusions, and pressure is concentrated on the projections of the wafer. Accordingly, the projections are removed at relatively high surface removal speed, resulting in uniform removal of the projections.
The types of slurry are roughly classified into a slurry for oxide, a slurry for metal, and a slurry for poly-silicon according to the type of object to be polished. The slurry for oxide is used to polish an interlayer insulating film and a silicon oxide (SiO2) layer employed in an STI (shallow trench isolation) process, and roughly comprises polishing particles, deionized water, a pH stabilizer, and a surfactant. The polishing particles function to mechanically polish the surface of the object by means of pressure from a polishing machine, and are exemplified by silica (SiO2), ceria (CeO2), and alumina (Al2O3). Particularly, ceria slurry is frequently used to polish the silicon oxide layer during the STI process, and in this case, a silicon nitride layer is mainly used as a polishing stopper layer.
If polishing speed selectivity of the oxide layer to the nitride layer is low, generally, a dishing phenomenon, in which an excessive volume of the oxide layer is removed, occurs due to the loss of adjacent nitride layer patterns. Thus, it is impossible to achieve uniform surface flattening. Hence, an additive is added to the ceria slurry to reduce the removal speed of the nitride layer so as to improve the polishing speed selectivity of the oxide layer to the nitride layer. However, the use of the additive is disadvantageous in that the removal speed of the oxide layer, as well as the removal speed of the nitride layer, is reduced. Furthermore, the polishing particles of the ceria slurry are typically larger than those of the silica slurry, and therefore scratch the surface of the wafer.
Accordingly, the slurry for STI CMP requires high selectivity and polishing speed, dispersion and micro-scratch stabilities, and narrow and uniform particle size distribution. Additionally, the number of large particles having the size of 1 μm or more must exist within a predetermined range.
With respect to conventional technology of producing the slurry for STI CMP, U.S. Pat. Nos. 6,221,118 and 6,343,976, granted to Hitachi Inc., disclose a method of synthesizing ceria particles and a method of producing a slurry having high selectivity using the same.
These patents describe characteristics of particles required in the slurry for STI CMP, the type of additives containing polymer, and the production method using them in very critical and wide ranges. Particularly, the patents suggest wide ranges of an average grain size, an average primary particle size, and an average secondary particle size. In another conventional technology, U.S. Pat. No. 6,420,269, granted to Hitachi Inc., discloses a method of synthesizing various ceria particles and a method of producing a slurry having high selectivity using the same. Furthermore, in the prior arts, U.S. Pat. Nos. 6,436,835, 6,299,659, 6,478,836, 6,410,444, and 6,387,139, which have been made by Showa Denko Co. Ltd. in Japan, disclose a method of synthesizing ceria particles and a method of producing a slurry having high selectivity using the same. These patents mostly describe the types of additives added to the slurry, effects due to them, and a coupling agent.
However, the average particle size and the particle size distribution of the slurry particles suggested by conventional technologies are problematic in that, since the particles are large, if the slurry particles are used to produce semiconductors having fine design rules, many micro-scratches may be formed. With respect to this, if the large particles are removed through simple filtering in order to form a slurry having no scratches, consumption of filters increases and reproducibility is poor.
Additionally, conventional technologies disclose only the average particle size of polishing particles constituting the slurry and the distribution of the polishing particles, but not the effects of the specific size distribution of the slurry particles within the given average particle size range on the removal rate of STI CMP or on the creation of scratches. As well, they do not disclose other characteristics of the slurry produced under process conditions generating the given size distribution. Further, they do not disclose the effects of processes of producing slurries having different properties or of conditions of slurries depending on the processes to CMP.